Combining theoretical description with experimental in situ studies on the effect of alkali additives on the structure and reactivity of vanadium oxide supported catalysts

doi:10.1016/j.cattod.2008.04.049

Catalysis Today

Volume 139, Issue 3, 30 December 2008, Pages 209-213

https://doi.org/10.1016/j.cattod.2008.04.049 Get rights and content

Abstract

A periodic model has been selected to model the structure and reducibility of vanadia/titania catalysts and the effect of alkali doping for Li, Na and K. Alkali interact with both the surface vanadia units and the support. The calculated changes on vanadia structure are consistent with the changes evidenced by Raman spectroscopy. Reducibility is modeled by adsorption of one hydrogen atom, forming hydroxyl groups with the bridging V–O–Ti present in the model. The adsorption energy decreases in the series Li > Na > K, in agreement with experimental TPR results.

Introduction

Vanadium oxide, as an active species, is present in most of the catalysts for selective oxidation of alkanes [1], [2], [3], which allows achieving a high productivity at moderate temperature. A rational model of catalysts needs information on the nature of the active centers and their role in the catalytic system. The catalytic properties of the supported vanadium oxide species for the selective oxidation processes are strongly affected by the vanadia loading, preparation method, nature of the support and type of promoter. The vanadium content determines type of the metal oxide species present on the surface. The surface VOx species occur in the isolated form at low loading evolving into polymerized forms when increasing the vanadia content until the monolayer coverage is reached. The segregate V₂O₅ crystalline phase dominates on the surface support above monolayer coverage. Among the catalysts promoters, alkali metals are usually mentioned as promoters for industrial catalysts since they provide a higher selectivity for partial oxidation reactions [4], [5]. The molecular structure of supported vanadium oxides and oxide supports (e.g. TiO₂) has been studied by different experimental techniques [6], [7], [8]. The theoretical calculation and modeling permit to describe the structure, spectra characteristics and reactivity [9], [10], [11], [12], [13], [14]. An interaction of the alkali cations with the oxide supports has been detailed studied by experimental [15], [16], [17] and theoretical [18], [19], [20] techniques. However, the role of alkali dopants on the structure and properties of supported vanadium oxide is not fully understood. Titania-supported vanadia catalysts exhibit a noticeable dependence on the preparation method or the coverage level of the support. Below monolayer coverage, supported species do not form the aggregates of the V-alkali-O phases. However, the evident interaction, which results in the weakening of the vanadyl group and in the changes of the reactivity exists [15], [16], [17].

The present work aims to provide a molecular description of the structure and reactivity of the vanadium oxide catalysts supported on titanium oxide and the importance of alkali doping on their structure and reactivity by a combined theoretical and experimental studies. The surface features have been determined by complementary studies of theoretical modeling (DFT) and experimental in situ Raman spectroscopy and temperature-programmed reduction (TPR). The structural and reactive features are contrasted with those described by theoretical calculation. The results obtained will be important for understanding the changes occurring during the catalytic act as well as will be complementary tools to describe the structures and activity observed during operando studies.

Section snippets

Computational studies

The periodic DFT calculations have been performed with the VASP code [21], [22] using the Perdew–Burke–Ernzerhof functional [23], [24] for the total energy determination. Spin-polarized calculations have been performed for the open shell systems (hydrogenated and alkali doped systems). The valence electrons have been described by plane waves based functions (cutoff energy = 400 eV) with the following electrons treated explicitly: O: s²p⁴, Ti: s²d², V: s²d³, M: s¹ (M = Li, Na), K: s¹p⁶. The core

Geometry

The presence of alkali atoms disturbs the vanadia–titania system geometry in different aspects. On the one hand, the alkali atoms form a strong ionic bond with both the vanadium and the support units. The alkali atom binds to two vanadium oxide units dispersed on the titania surface in our model. In accordance to this model, alkali atom forms bonds with a bridging oxygen O_B in V–O–Ti and the other one to a terminal oxygen O_T in V double bond O group, as is presented in Fig. 1. The M–O distances obtained

Conclusions

The combination of theoretical and experimental studies describes in a consistent way the effect of the alkali additives on the titania-supported vanadium oxide catalysts. The alkali atoms bind to both vanadia units and support forming strong ionic bonds. The vanadia/titania materials modified by alkali atoms possess a lower reducibility than the undoped catalyst. The bridging oxygen V–O–Ti is more reactive than the terminal oxygen of vanadyl group (V double bond O). It determines – a priori – which can be

Acknowledgements

This work has been supported by the COST ACTION D36 (Working Group D36-006-06) and has been accomplished in the frame of the French GDR “Dynamique moléculaire quantique appliqué à la catalyse”. Computational facilities by CCRE and IDRIS are acknowledged. Support was provided by the Spanish Ministry of Education and Science CTQ2005-02802/PPQ. AEL acknowledges Spanish Ministry of Education and Science for a “Juan de la Cierva” postdoctoral position, and ELD acknowledges Comunidad de Madrid

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Combining theoretical description with experimental in situ studies on the effect of alkali additives on the structure and reactivity of vanadium oxide supported catalysts

Abstract

Introduction

Section snippets

Computational studies

Geometry

Conclusions

Acknowledgements

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